PROJECT SUMMARY/ABSTRACT Skeletal muscle plays a fundamental role in health maintenance. Loss of muscle mass can have long-lasting deleterious consequences, ranging from prolonged periods of convalescence to premature death. Treatments to prevent muscle loss are a major unmet clinical need relevant for a wide range of conditions including inflammatory, endocrine, infectious, neurological and nutritional disorders. This is a consequence of the poor understanding of the molecular basis of muscle size control. Muscle growth and maintenance is largely determined by the ability of the muscle to synthesize proteins, which in turn, is regulated by muscle ribosome content. Ribosome production is mainly controlled by transcription of ribosomal (r)DNA genes by RNA Polymerase I (Pol I). We have recently demonstrated that disrupting Pol I transcription prevents muscle hypertrophy. This strongly supports that idea that an increase in the ribosomal capacity of the muscle is necessary for hypertrophy. In this proposal, we seek to understand whether removal of repressors of Pol I function can enhance ribosomal production and stimulate muscle hypertrophy. We developed a novel model where genetic removal of the Pol I repressors Rb and p130, results in enhanced rDNA gene expression, increased ribosome production, and is sufficient for skeletal muscle hypertrophy. This model supports our hypothesis, and highlights a new paradigm of muscle growth control where the de-repressing the ribosomal machinery leads to a hypertrophic response. In this proposal, we will define novel mechanisms controlling muscle rDNA gene expression by studying the role of transcription and chromatin remodeling factors exerting modulatory functions on rDNA genes. We propose three specific aims where we will: 1) define the molecular mechanisms by which simultaneous removal of Rb and p130 enhances ribosome production and muscle hypertrophy, 2) identify muscle-specific transcriptional regulation of rDNA genes during hypertrophy, and 3) determine the epigenetic mechanism controlling ribosome production during muscle hypertrophy. These studies are highly relevant for human health because they will advance our knowledge of how muscle mass is regulated, and this is fundamental for the development of therapies aimed at maintaining and restoring skeletal muscle mass and function.